Exhaust gas purifying systems using a pollution control device to purify the exhaust gas of an internal combustion engine, are well known. Pollution control devices include catalytic converters and exhaust gas filters (e.g., diesel particulate filters). For example, it is well known to use ceramic catalytic converters to purify an exhaust gas containing carbon monoxide (CO) and hydrocarbon (HC) exhausted from the engine of automobiles. In general, a pollution control device includes a pollution control element mounted in a housing. For example, a ceramic catalytic converter basically contains a honeycomb catalyst support (catalyst element) made of ceramic in a casing, i.e., housing made of metal.
Generally, the space between the pollution control element and the housing is filled with a thermal insulating material. For example, in a catalytic converter, the space between the catalyst support and the casing is filled with a thermal insulating material consisting typically of inorganic fibers and organic fibers and/or a liquid or pasty organic binder such as, for example, that disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-61686, 59-10345 or 61-239100. As a result, the thermal insulating material, with which the space is filled, retains the catalyst support, thereby making it possible to prevent a mechanical shock caused by impact and vibration, or a thermal shock caused by heat cycles from giving to the catalyst support. According to a catalytic converter with such a constitution, a desired operation can be realized because breakage or detrimental movement of the catalyst support does not occur.
The above-described exhaust gas purifying system is generally provided with an oxygen sensor and it controls the concentration of oxygen in the exhaust gas, thereby to effect optimum purification of the exhaust gas through a catalytic converter. It is considered to be preferred that the catalytic converter is operated at a higher temperature so as to improve purification of the exhaust gas and to improve the fuel cost. With the recent tightening of exhaust gas control due to protection of the global environment, further purification of the exhaust gas due to an increase in operating temperature tends to be effected. On the other hand, a nitrogen oxide (Nox) tends to be easily evolved in the exhaust gas as a result of an increase in operating temperature. Accordingly, it has been known that an air fuel ratio due to an accurate signal from an oxygen sensor is important in the exhaust gas purifying system.
However, the thermal insulating material as disclosed in the above publications (Kokai) can not be easily used, together with a high sensitivity oxygen sensor. The reason is as follows. That is, since an organic matter such as organic fibers and/or organic binder are added in the above thermal insulating material in a comparatively large amount, i.e. about 4 to 50% wt %, based on the total amount of the thermal insulating material, the organic matter is likely to be easily incorporated into the exhaust gas on initial operation of the catalytic converter, thereby to cause wrong operation of the exhaust gas purification system.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 11-166414 discloses a thermal insulating material comprising crystalline alumina fibers as a principal component, wherein the content of an organic matter is reduced to 1 wt % or less. However, since the surface of this thermal insulating material is covered with a protective sheet, an additional complicated step of removing the protective sheet must be carried out after the thermal insulating material was disposed between a casing and a catalyst support so as to apply it to a catalytic converter. There is also a problem about disposal of the removed protective sheet.
Japanese Unexamined Patent Publication (Kokai) No. 6-239656 discloses a multi-layer structure thermal insulating material. This publication teaches to produce the multi-layer structure thermal insulating material by using a very small amount of a polymer aggregating agent of an organic matter. However, a complicated step of laminating a plurality of mats must be required because of a multi-layer structure of the thermal insulating material.
Japanese Unexamined Patent Publication (Kokai) No. 7-286514 discloses a holding seal material (corresponding to a thermal insulating material) made by treating a laminated material of inorganic fibers such as crystalline alumina fibers using a needle punching method. The holding seal material is likely to cause protrusion or scattering of inorganic fibers from the surface of the insulating material, although the organic matter is removed by calcining the laminated material. As a result, the end portion of the inorganic fibers tends to be directly contacted with the operator to impart an unpleasant feeling to him on incorporation of the above holding seal material into the catalyst converter.
As described above, various thermal insulating materials have hitherto been suggested to apply to a catalytic converter, however, still some room for improvement is left in any of thermal insulating materials.